It’s a three way tie between:
Holy Mount Athos, Greece
If I absolutely had to choose one, I’d choose Jerusalem grudgingly.
I’d also love to visit other ancient centers of Christianity:
Constantinople (ehrrm, Istanbul), Turkey
How awesome of a trip would this be: Rome, to Mt Athos, to Constantinople, then over to Iraq, over to Antioch, then down to Jerusalem, then finally down to Alexandria.
3 Continents, 6 countries, 7 locations, covering all the most important major ancient centers of Christendom.
Ahhh, a man can dream!